(1) Field of the Invention
The present invention relates to refractory purging plugs generally used for blowing gas into a metallurgical vessel. It refers in particular to such purging plugs provided with a wear indicator informing an operator of the level of wear of the purging plug.
(2) Description of the Related Art
In metal forming processes, metal melt is transferred from one metallurgical vessel to another, to a mould or to a tool. For example a ladle is filled with metal melt out of a furnace and transferred to a tundish. The metal melt can then be cast from the tundish to a tool for forming slabs or to a mould for forming billets or ingots. In some cases, it is desirable to blow a gas into the molten metal contained in such metallurgical vessels. This can be useful to accelerate the homogenization of the temperature and composition of a bath, to carry non metallic inclusions present in the bulk of the bath up into the slag top layer, to create favourable conditions within the molten metal, and the like. The gas is generally blown into the molten metal by means of purging plugs located at the bottom or side of a metallurgical vessel such as a ladle or a tundish.
Purging plugs are in the form of a block of refractory material, generally extending along a longitudinal axis. At one end of the block, a gas inlet connected to a source of pressurized gas is fluidly connected to a gas outlet at the opposite end of the block. The gas inlet and gas outlet may be fluidly connected to one another through an open pore network, by one or more channels (e.g., slit shaped or with circular cross-section), or a combination of both. An open pore network is sometimes said to yield “indirect permeability,” whilst a channel is said to yield “direct permeability.” It is generally recognized that direct permeability plugs are more efficient than indirect permeability plugs, mostly because a pore network comprises an uncontrollable tortuosity which affects negatively the permeability of the plug, whilst the size and geometry of a manufactured channel can be controlled such as to minimize tortuosity, and therefore increase the permeability compared with pores of same equivalent diameter or dimensions.
As illustrated in FIG. 1, a purging plug (1) is usually embedded in the wall and lining of a metallurgical vessel (31), with the gas inlet facing the exterior side of the metallurgical vessel, and with the gas outlet facing the inside of the vessel, in contact with the molten metal. The terms “gas inlet” and “gas outlet” are defined with respect to the flow direction (11) of the gas being injected into the metallurgical vessel. Because of their structure and extreme working environment, purging plugs wear more quickly than the refractory liner of the vessel, with severe erosion of the order of several mm or even cm after each use. This means that during the lifetime of a metallurgical vessel such as a ladle, gas plugs need be changed several times. The changing of a gas plug takes time, is work intensive, and requires the purchase of a new plug each time, so that operators tend to push the use of a plug as long as possible to extend the intervals between plug changes. One major danger with pushing the use of a plug too long, is that if the erosion of the plug is too deep, the remaining base of the plug may be unable to resist the pressure of the molten metal and may leave a gaping hole whence molten metal may flow out freely. If this happens during transfer of the ladle towards a tundish, it may spray molten metal at temperatures of the order of 1400° C. all over the workshop with dramatic consequences. To avoid this to happen, wear indicators have been proposed in the art, informing the operator of the degree of erosion undergone by a purging plug, who can decide whether it could be used again or not.
U.S. Pat. No. 5,202,079 proposes an indirect permeability type plug (i.e., wherein the gasflow path is defined by the porosity of the plug) comprising an outer body defining the external geometry of the plug, said outer body being made of a non-porous refractory material, and an inner core made of a refractory material of higher porosity, allowing gas to flow from an inlet to an outlet of the plug. The transverse cross section of the porous core, normal to the longitudinal axis of the plug, varies along said longitudinal axis. When a metallurgical vessel is emptied of its molten metal load, gas is injected through the plug as it is still hot, and the gas flowing out of the hot plug into the interior of the empty vessel will glow defining the shape of the porous core cross-section exposed to the interior of the vessel giving the operator an indication on the level of erosion of the plug depending on the shape of the glowing section. This system, however, is restricted to indirect permeability type plugs, and reduces the efficacy of the plug by restricting the gas flow path to the inner core of the plug. Another disadvantage of this type of plug is the cooling effect of the gas. The plug gets colder. This increases the wearing but also the risk of metal freezing and clogging of the plug.
Similarly, U.S. Pat. No. 4,385,752 discloses porous plugs comprising a porous outer body and a porous inner core having a different emissivity than the refractory of the outer body. The principle is therefore quite similar to the previous document, with the difference that the outer body is also porous, thus increasing the efficacy of the plugs with respect to the one disclosed in U.S. Pat. No. 5,202,079. This solution is, however, also restricted to porous plugs only.
U.S. Pat. No. 5,249,778 extends the principle disclosed in the former two documents to direct permeability plugs, by providing a plug with one or more channels extending from a gas inlet to a gas outlet, and further including a porous insert in fluid communication with the gas inlet, and extending along the longitudinal axis of the plug up to the height corresponding to, or nearly to the end of use of the plug. When erosion reaches the porous insert, gas flowing through the porous insert will cool the refractory centre quicker than the periphery, thus creating a dark spot at the centre indicative of the end of the plug's service life. Each of the foregoing plugs require gas to be injected through the plug when the vessel is empty, and therefore not necessarily close to a connection to a gas source. The cooling of the plug leads to the drawbacks above mentioned.
U.S. Pat. No. 5,330,160 discloses a purging plug comprising an insert made of a material having a lower melting point than the metal contained in the vessel, said insert being inserted into a cavity extending from the plug top (which is to contact the molten metal) down to a level of plug considered as indicative of the end of the service life thereof. The low melting point insert can extend up to and is flush with the top end of the plug, or end to a level lower than said top end, the top of the cavity being filled with a top cap made of a high wear resistant refractory material. When the top cap is worn out and the top of the low melting temperature material contacts the molten metal to be cast, the low melting temperature material melts and is replaced in the cavity by molten metal to be cast. When the vessel is emptied, some metal remains in the cavity and glows forming a “magic eye” clearly visible by an operator. When the erosion of the plug reaches the bottom of the cavity, the magic eye disappears and the operator is thus informed that the plug should be replaced. In a variation of the former plug, U.S. Pat. No. 5,421,561 discloses a plug wherein the low melting temperature insert is enclosed in a non-metallic tube acting as thermal insulator to further enhance the glow of the “magic eye”. The manufacturing of such plug is rather work intensive, as a cavity needs be drilled into the body of the plug and the insert inserted therein, whilst the space between the cavity walls and the insert must be decreased. One wonders whether the low melting temperature visual wear indicator is needed at all, since all is required is a cavity. Furthermore, this system provides a binary signal, indicative that the plug can be used as long as the magic eye is visible, but it does not inform the operator on the erosion rate of the plug. In practice, to be on the safe side, the operators replace the plug when the magic eye appears.
The present invention proposes a solution allowing to estimate the erosion rate of the plug, which is very easy and relatively cheap to manufacture.